In 2014, the worst Ebola outbreak in history erupted in West Africa. Hardest hit were Guinea, Liberia and Sierra Leone, which together totaled 14,432 laboratory confirmed cases and 9,936 deaths as of March 7, 2015. The outbreak touched Nigeria (20), as well as Senegal (1), Mali (8), Spain (1), the U.K. (1) and the U.S. (4) [source: CDC].

For a disease that isn't that easily transmitted (it requires direct contact with infected blood or bodily fluids, although some in Africa may contract it by handling bush meat or infected bats), Ebola certainly inspires terror. Perhaps we fear the case fatality rate, which ranges from 25-90 percent, depending on the outbreak, and averages around 50 percent. Likely, the lack of any cure or quick, reliable test has compounded these fears. And then there's the horror of the disease's progression, which begins two to 21 days after exposure (eight to 10 days average) and includes unexplained hemorrhaging, fever, severe headache, muscle pain, weakness, diarrhea, vomiting and abdominal pain. And finally, there are those fears, stoked by frightful films and panicked pundits, that the disease will mutate into an airborne strain.

Whatever the reason, the fight against Ebola faces a further challenge: the central mystery over the virus's natural reservoir host. Experts suspect bats but, until they know for sure, we cannot predict how or where the virus will show up in humans and spawn an outbreak [source: CDC].